Systems and methods for sensing properties of a workpiece and embedding a photonic sensor in metal

ABSTRACT

Systems and methods for sensing properties of a workpiece and embedding a photonic sensor in metal are disclosed herein. In some embodiments, systems for sensing properties of a workpiece include an optical input, a photonic device, an optical detector, and a digital processing device. The optical input provides an optical signal at an output of the optical input. The photonic device is coupled to the workpiece and to the output of the optical input. The photonic device generates an output signal in response to the optical signal, wherein at least one of an intensity of the output signal and a wavelength of the output signal depends on at least one of thermal characteristics and mechanical characteristics of the workpiece. The optical detector receives the output signal from the photonic device and is configured to generate a corresponding electronic signal. The digital processing device is coupled to the optical detector and determines at least one of the thermal characteristics and mechanical the characteristics of the workpiece based on the electronic signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national phase application under 35 U.S.C.§371 of International Patent Application No. PCT/US2007/000133, filedJan. 3, 2007 and entitled “Systems and Methods for Sensing Properties ofa Workpiece and Embedding a Photonic Sensor in Metal,” which claimspriority to U.S. Provisional Patent Application No. 60/755,799, filedJan. 3, 2006 and entitled “Distributed Subwavelength Micro- andNano-photonics for Ultrahigh Spatial- and Temporal-Resolution inDisplacement, Strain, Vibrational, and Thermal Sensing,” the entirecontents of each of which are incorporated herein by reference.

STATEMENT OF GOVERNMENT INTEREST

The present subject matter was developed with Government support undergrant number 0528900 awarded by the National Science Foundation. TheGovernment may have certain rights in the present subject matter.

TECHNOLOGICAL FIELD

The disclosed subject matter relates to systems and methods for sensingproperties of a workpiece and embedding a photonic sensor in metal.

BACKGROUND

Placing temperature and strain sensors directly into a manufacturingenvironment can help to obtain effective monitoring and control ofmanufacturing processes for computer chip and mechanical products. Ifcritical conditions in these processes are continuously monitored withsensors, problems can be detected and solved during the processingcycle, resulting in improved product quality and productivity. There areongoing efforts to fabricate electrically based micro-sensors, such asthin film thermocouples and strain gauges, for in-situ manufacturingprocess monitoring. Typically, however, with electrically based sensorarrays having a large number of sensors, e.g., that are distributedacross a computer chip or mechanical product, the assembly of wires thatallow readouts from these arrays can be cumbersome and costly.Monitoring the fabrication of metal structures in hostile manufacturingenvironments is particularly challenging because the presence of hightemperatures, corrosive agents such as acids, alkalis, and oxidizers,and electromagnetic interference can damage an electrical sensor orimpair its ability to monitor processes.

SUMMARY

Systems and methods for sensing properties of a workpiece and embeddinga photonic sensor in metal are disclosed herein.

In some embodiments, systems for sensing properties of a workpieceinclude an optical input, a photonic device, an optical detector, and adigital processing device. The optical input provides an optical signalat an output of the optical input. The photonic device is coupled to theworkpiece and to the output of the optical input. The photonic devicegenerates an output signal in response to the optical signal, wherein atleast one of an intensity of the output signal and a wavelength of theoutput signal depends on at least one of thermal characteristics andmechanical characteristics of the workpiece. The optical detectorreceives the output signal from the photonic device and is configured togenerate a corresponding electronic signal. The digital processingdevice is coupled to the optical detector and determines at least one ofthe thermal characteristics and mechanical the characteristics of theworkpiece based on the electronic signal.

Some embodiments include one or more of the following features. Thedigital processing device also modifies a parameter in a process beingapplied to the workpiece in response to the at least one of the thermalcharacteristics and the mechanical characteristics of the workpiecedetermined by the digital processing device. The photonic deviceincludes a microring resonator. The photonic device includes a defect ina photonic crystal. The workpiece is one of a computer chip and amechanical product. The photonic device is embedded in a layer of theworkpiece. The photonic device has a Q between approximately 100 andapproximately 10⁵. The photonic device has a Q between approximately2,000 and approximately 20,000. The optical input includes a laser. Theoptical detector includes a photodiode. The optical input is coupled tothe photonic device by a waveguide.

In some embodiments, methods of embedding photonic sensors in a metalinclude: depositing a first optically insulating layer on a substrate;depositing and patterning a waveguide material on the first opticallyinsulating layer to define the photonic sensor; depositing andpatterning a second optically insulating layer over the photonic sensor;depositing a first metal layer over the second optically insulatinglayer; etching the substrate to free the first and second opticallyinsulating layers, the photonic sensor, and the first metal layer fromthe substrate and thus expose a surface of the first opticallyinsulating layer; depositing a second metal layer over the exposedsurface of the first optically insulating layer and thus substantiallyembed the photonic sensor and first and second optically insulatinglayers between the first and second metal layers.

In some embodiments, patterning the waveguide material includes at leastone of lithography and etching.

In some embodiments, methods for sensing properties of a workpiece witha photonic device include: providing an optical signal to the photonicdevice; coupling the photonic device to a workpiece for which ameasurement is desired, the photonic device generating an output signalin response to the optical signal, wherein at least one of an intensityof the output signal and a wavelength of the output signal depends on atleast one of thermal characteristics and mechanical characteristics ofthe workpiece; receiving the output signal from the photonic device, andbased on the output signal determining at least one of the thermalcharacteristics and the mechanical characteristics of the workpiece.

Some embodiments include one or more of the following features.Modifying a parameter in a process being applied to the workpiece inresponse to the at least one of the thermal characteristics and themechanical characteristics of the workpiece determined. Coupling thephotonic device to the workpiece includes embedding the photonic devicein a layer of the workpiece.

In some embodiments, systems for sensing properties of at least tworegions of a workpiece with at least two distributed photonic devicesinclude an optical input, at least two distributed photonic devices, anoptical detector, and a digital processing device. The optical inputoutputs an optical signal. The at least two distributed photonic devicesare coupled to the at least two regions of the workpiece and to theoptical input. The at least two distributed photonic devices generate atleast two output signals in response to the optical input, wherein atleast one of an intensity and a wavelength of each of the at least twooutput signals depends on at least one of thermal characteristics andmechanical characteristics of the at least two regions of the workpiece.The optical detector receives the at least two output signals from theat least two distributed photonic devices and is configured to generatea corresponding electronic signal. The digital processing device iscoupled to the detector and determines at least one of the thermalcharacteristics and the mechanical characteristics of the at least tworegions of the workpiece based on the electronic signal.

Some embodiments include one or more of the following features. Anoptical waveguide that couples the optical input to the at least twodistributed photonic devices. An optical waveguide that couples the atleast two distributed photonic devices to the optical detector. The atleast two distributed photonic devices are embedded in one or morelayers of the workpiece. At least one of the photonic devices includes amicroring resonator. At least one of the photonic devices includes adefect in a photonic crystal.

In some embodiments, systems for sensing properties of a workpieceinclude: means for outputting an optical signal; means, coupled to theworkpiece and to the optical signal, for generating an output signal inresponse to the optical signal, wherein at least one of an intensity ofthe output signal and a wavelength of the output signal depends on atleast one of thermal characteristics and mechanical characteristics ofthe workpiece; means for detecting the output signal; and means fordetermining at least one of the thermal characteristics and themechanical characteristics of the workpiece based on the detected outputsignal.

In some embodiments, the means for generating an output signal includesone of a microring resonator and a defect in a photonic crystal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a micrograph of a nanocavity according to some embodimentsof the disclosed subject matter.

FIG. 1( b) is a micrograph of a nanophotonic crystal sensor according tosome embodiments of the disclosed subject matter.

FIG. 2( a) is a schematic of an on-chip sensor system according to someembodiments of the disclosed subject matter.

FIG. 2( b) shows micrographs of perspective and plan views of amicroring resonator according to some embodiments of the disclosedsubject matter.

FIG. 2( c) is a plot of the normalized transmission peak of themicroring resonator of FIG. 2( b) according to some embodiments of thedisclosed subject matter.

FIG. 3( a) shows micrographs of microring resonators according to someembodiments of the disclosed subject matter.

FIG. 3( b) is a schematic of a microring resonator according to someembodiments of the disclosed subject matter.

FIG. 4 is a schematic of a collection of information from photonicsensor arrays according to some embodiments of the disclosed subjectmatter.

FIG. 5 illustratively shows a method of embedding photonic sensors in ametal according to some embodiments of the disclosed subject matter.

FIGS. 6( a) and 6(b) are micrographs of optical sensors embedded inmetals according to some embodiments of the disclosed subject matter.

FIG. 6( c) is a micrograph of a thermocouple embedded in metal accordingto some embodiments of the disclosed subject matter.

FIG. 7 shows images of the transfer of a batch of photonic sensors to asubstrate and the embedding of the sensors in metal according to someembodiments of the disclosed subject matter.

FIG. 8 is a schematic of a system for separating mechanically andthermally induced strain components according to some embodiments of thedisclosed subject matter.

DETAILED DESCRIPTION

Systems and methods for sensing properties of a workpiece and embeddinga photonic sensor in metal are disclosed. In some embodiments, photonicdevices, such as microring and nanophotonic crystal resonators, areuseful for monitoring and controlling manufacturing processes for piecessuch as computer chips and mechanical products, by monitoring theeffects of thermo-mechanical phenomena arising from the manufacturingprocesses on the pieces. More specifically, in some embodiments and asdescribed in greater detail below, the photonic sensors can beincorporated into and distributed throughout the pieces during an earlymanufacturing stage, e.g., before the pieces are exposed to processesthat would benefit from being monitored. The sensors can be madesufficiently small, and can be selectively placed, such that theygenerally will not interfere with normal operation of the finishedpiece. In some embodiments, for example, the nanophotonic crystal-basednanocavities described below, the sensors can be less than about 1micron in size, e.g., between about 100 nm and 1 micron in size, i.e.,sub-micron, while in other embodiments, for example the microringresonators described below, the sensors can be between about 1 micronand about 10 microns in size, although other sizes are possible. Alongwith the sensors, optical waveguides can also be incorporated into anddistributed throughout the pieces. These waveguides can allow an opticalinput of known wavelength distribution, from an optical source such as alaser, to be introduced to the sensors. The waveguides can also allow acorresponding optical output that relates to the thermo-mechanicaleffects of a process on the pieces to be collected from the sensors by adetector. Based on the intensity and/or wavelength distribution of theoptical output, a computer processor in communication with the detectorcan then appropriately modify the manufacturing process so as to adjustthe effects of the process on the pieces, for example, so as to inhibitdamage to the pieces or otherwise improve the process.

The photonic sensors can provide multiple features that are difficult toobtain with conventional electrically based sensors. First, because thephotonic sensors have a significantly smaller physical mass and thermalmass than conventional sensors, they can provide an enhanced ability tosense process changes (e.g., strain, temperature, and vibration). Thisspeed of sensing allows those processes to be adjusted as needed.Photonic sensors are also highly resistant to electromagneticinterference and to hostile environments that can render typicalelectrically based sensors inoperable. Photonic sensors also permit thecollection of data with spatial resolution and sensitivity that isgreatly enhanced over conventional macro-sensors, for example, allowingspatial resolution on the order of 10 μm or better, strain resolution onthe order of 10⁻⁵, and temporal resolution on order of a nanosecond orbetter. The output from multiple arrays of sensors can also bemultiplexed, as discussed in greater detail below. Photonic sensors canalso be embedded into metals to improve sensor survivability andreliability in manufacturing environments. Metal embedded photonicsensors can be fabricated with a batch fabrication process, andsubsequently transferred into larger metallic structures inmanufacturing environments. Minimally invasive physical protection(e.g., thermal, photonic, and/or mechanical protection) can be added tothe embedded photonic sensors to improve their functionality. Thesensors can further be characterized after embedding, and calibratedunder dynamic thermal and mechanical loads.

One suitable kind of photonic sensor is based on resonators innanophotonic crystals. Nanophotonic crystals are periodic lattices ofone or more alternating dielectric materials that permit photonic bandgaps. These structures are analogous to the atomic potential latticesleading to electronic band gaps in semiconductors. “Defects” in theperiodicity of nanophotonic crystals provide an exceptional means todesign nanoscale optical resonators from first principles. Morespecifically, an optical resonator arising from these defects inperiodicity resonates at an optical frequency that is highly sensitiveto the resonator's thermo-mechanical environment. Thus, monitoring theresonant optical frequency of the optical resonator provides informationabout that environment that can be used to modify that environmentappropriately, e.g., by changing a process parameter.

FIG. 1( a) is a scanning electron micrograph of a nanocavity 120 withina nanophotonic crystal 100, in accordance with some embodiments. Thenanocavity can be used as a sensor for thermally and/or mechanicallyinduced strain. The dashed line around nanocavity 120 indicates thedefect region where the periodicity of the nanophotonic crystal isdisrupted. This defect region forms a nanoscale optical resonator thathas a characteristic operating wavelength λ_(o). Light at an integermultiple of this characteristic wavelength λ_(o) resonates within thecavity, and experiences a relatively low loss. As discussed in greaterdetail below, the presence of thermally and/or mechanically inducedstrain modifies the characteristic operating wavelength and the loss ofthe cavity, and thereby allows this strain to be opticallycharacterized.

The resonator can be characterized by its quality factor Q, which is ameasure of its extrinsic and intrinsic optical losses. The resonator Qis defined as λ₀/δλ, which is the ratio of the resonator operatingwavelength λ_(o) to the resonator full-width half-maximum (FWHM) δλ. Atthe resonant wavelength, the Q of a nanophotonic resonator is as high as10⁵, which is two to three orders of magnitude higher than conventionalfiber Bragg sensors, which can also be used for temperature and strainsensing in manufacturing environments.

Bragg sensors generally contain Bragg gratings as a sensing element.Application of mechanical strain or a temperature change to thesegratings shifts the center wavelength of light reflected from the fiberBragg gratings, and the wavelength shift provides the necessaryinformation to calculate the change in strain or temperature. However,Bragg sensors are typically significantly larger than photonic sensors,e.g., between about 125 and 250 μm in size, versus less than about 10 μmin size for some embodiments of photonic sensors, and are significantlyless sensitive as discussed in greater detail herein.

The optical response time of a nanophotonic resonator, e.g., the amountof time it takes a given amount of light resonating within a nanocavityto decay below a threshold value, provides a measure of the rate atwhich continuous strain event monitoring can be achieved. However, achange in the temperature of the resonator, or the application of amechanical force, such as compression of the resonator, can cause astrain that modifies the physical characteristics of the structure. Thiscan change the resonator Q as well as the resonant wavelength of thecavity. Thus, a measurement of the resonator Q can be a measure oflocalized strain.

In some embodiments, the Q of the resonator, and, thus, a measurement ofthe thermal and/or mechanical strain experienced by the resonator, canbe determined interferometrically. For example, in some embodiments,“signal” light of a first wavelength λ₂ and “control” light of a secondwavelength λ₁ are transmitted into the crystal 100 via a waveguide 140adjacent the nanocavity 120. The first wavelength λ₂ is selected to besufficiently close to the resonant wavelength of the nanocavity 120 thatit couples into the nanocavity 120. The light at the first wavelength λ₂interacts with the cavity according to the Q of the cavity, and couplesback into the waveguide 140. The light at the second wavelength 2 doesnot significantly couple into the nanocavity 120, but instead simplytransmits along the waveguide 140. The light at the first and secondwavelengths λ₂ and λ₁ interfere with each other, and the intensity ofthis interference is related to how much attenuation the firstwavelength λ₂ experienced due to its interaction with the nanocavity120, and thus is related to the strain within the nanocavity. Thisinterferometric intensity is proportional to 1/Q. An optical detector(not shown) in communication with the waveguide and capable of detectingthe change in interferometric intensity between λ₂ and λ₁ arising fromthe strain within nanocavity 120 converts the optical interferencesignal into an electrical signal. This electrical signal is transmittedto a computer processor (not shown) that uses that information tocalculate the strain within nanocavity.

When integrated onto a silicon chip, these nanophotonic crystalresonators, like the microring resonators described below, allow thethermo-mechanical strain experienced by the silicon chip to be monitoredwith enhanced spatial and temporal resolution. The large resonator Q'sallow distinguishable Lorentzian resonance peaks under small strain orthermal loading, permitting improved sensitivities.

Furthermore, due to the 1-dimensional geometry of these nanophotoniccrystal resonators, strain directionality can be obtained. For example,FIG. 1( b) shows a nanophotonic crystal sensor 130 with sensing in boththe x and y directions (both ε_(x) and ε_(y)) from the same nanocavityresonator 150 in accordance with some embodiments. Sensor 130 includeswaveguide 160 in the x-direction, which transmits “signal” and “control”light at first and second wavelengths, respectively, into nanocavityresonator 150 in the x-direction. As discussed above for the sensor ofFIG. 1( a), the “signal” light at the first wavelength couples intonanocavity 150, and then couples back into waveguide 160. The “signal”light interferes with the “control” light at the second wavelength, andthe intensity of this interference is related to the strain experiencedby the nanocavity 150 in the x-direction. A first detector (not shown)detects the interferometric intensity, and a computer processor inelectrical communication with the detector analyzes the resultingsignal. Sensor 130 also includes waveguide 170 in the y-direction, whichtransmits “signal” and “control” light at third and fourth wavelengths,respectively, into nanocavity resonator 150 in the y-direction. The“signal” light at the third wavelength couples into nanocavity 150, andthen couples back into waveguide 170. The “signal” light interferes withthe “control” light at the fourth wavelength, and the intensity of thisinterference is related to the strain experienced by the nanocavity inthe y-direction. A second detector (not shown) detects theinterferometric intensity, and the computer processor, which is also inelectrical communication with the second detector, analyses theresulting signal. The signals in the x- and y-directions are effectivelyindependent of each other, allowing the separate measurements of thenanocavity's strain in the x- and y-directions with minimal cross-talk.

Another suitable kind of photonic sensor is based on microringresonators. Microring resonators are traveling wave resonators with atransmission response that follows a Lorentzian line shape filter. FIG.2( b) shows micrographs of a perspective view (top image) and plan view(bottom image) of a fabricated microring resonator 210 in accordancewith some embodiments. Signal light from an optical source (not shown),e.g., a fiber optic source, travels along an input waveguide 230 that ispositioned adjacent to the microring resonator 210. The microringresonator 210 becomes a resonant sink for the signal light when anintegral number of wavelengths match its optical circumference, i.e., ata resonant frequency of the resonator. Light that couples from the inputwaveguide into the resonator then couples from the microring resonator210 into an output waveguide 220. This coupled light then travels alongthe waveguide to an optical detector (not shown), e.g., a photodiodewith fiber optic input. The electronic detector output is optionally fedto a lock-in amplifier to reduce noise, and is then input to a computerprocessor for analysis. FIG. 2( c) is a plot 240 of the normalizedtransmission peak of the microring resonator of FIG. 2( b), as monitoredat a detector in accordance with some embodiments. Plot 240 shows tworesonance frequency peaks of the microring resonator, and the distancebetween the peaks corresponds to the free spectral range of theresonator. As discussed in greater detail below, the central wavelengthand bandwidth of the transmission peak are related to the strain themicroring resonator experiences due to thermo-mechanical phenomena.

FIG. 2( a) is a schematic of an on-chip fabricated sensor system 200 inaccordance with some embodiments. System 200 includes a microringresonator 211, an input waveguide 231, an output waveguide 221, a fiberoptic detector 222, and a fiber optic source 232. Optionally, asecondary detector 223, in optical communication with microringresonator 211 via third waveguide 224, can be used for additionaldetection, e.g., for monitoring the complementary transfer functionoutput, as a back-up to the detector fiber. This complementary transferfunction output includes signal light that does not couple into themicroring resonator, the intensity of which is reduced by the amount oflight that does couple into the microring resonator, thus providing asecond method of measuring the change in optical intensity due tooptical coupling of signal light into the resonator. V-grooves 260 canalso be wet-etched anisotropically on the chip, using standard CMOSprocesses, to facilitate reliable alignment and placement of the fiberoptic source 232 and detectors 222, 223 on the chip relative to theappropriate waveguides 231, 221, and 224, respectively.

When a chip, having the embedded sensor system 200, is mechanically orthermally compressed or stretched, changes in the output signalsrecorded by the detector(s) can be tracked on GHz-frequency (nstime-scale) commercial optical detectors, and monitored continuously andsimultaneously at numerous network grids of the manufacturing structurethrough wavelength-division multiplexing. The on-chip sensors can alsobe thermally loaded to finely map-out thermal variability and diffusionresponses on small lengthscales.

The change in the resonant frequency Δλ, of the microring resonator,with respect to applied strain ε, is:

$\begin{matrix}{\frac{\Delta\lambda}{\lambda_{o}} = {{n_{eff}\left\lbrack {1 - {\frac{1}{2}(ɛ)n_{eff}^{3}}} \right\rbrack}ɛ}} & (1)\end{matrix}$

where λ_(o) is the resonant frequency, n_(eff) an effective refractiveindex, and

(ε) is the photoelastic coefficient as a function of strain. For strainson the order of 0.1%, the photoelastic correction term

$\frac{1}{2}(ɛ)n_{eff}^{3}$

can be negligible, and the change in resonant frequency Δλ is a linearfunction of strain ε. The ratio (Δλ/λ_(o)) is a linear function oftemperature variations, and related as: Δλ/λ_(o)=n_(eff)(α+β)ΔT/T, whereα is the linear thermal expansion coefficient and β the photothermalcoefficient expressing dependence of refractive index on temperature.

The ring resonator can be separated from the waveguides by a gap δ,which is designed and fabricated to be approximately equal to thecritical coupling distance for the resonator. A gap of 300 nm isillustrated in the inset 310 of FIG. 3( a). The coupling coefficient κ(the spatial overlap integral between the resonator and waveguide modes)is exponentially dependent on the coupling gap δ, and at a “critical”coupling distance, the coupling of light between the waveguide andresonator is the largest. The value of the critical coupling distancedepends, among other things, upon the resonant frequency of the ringresonator.

The presence of mechanical or thermal strain in the material changes thesize of the coupling gap δ, and therefore also changes the value of thecoupling coefficient κ. The resonator Q is related to κ by

${Q = \frac{2\pi^{2}{Rn}_{e}}{\lambda_{o}\kappa^{2}}},$

where R is the physical radius of the microring. Thus, a change in thegap size δ (and therefore the value of κ) leads to a large shift in theresonator Q. This change in Q can be measured as a change in the lightrecorded by the optical detector(s), and from that change the strain canbe determined. This provides a method for high-resolution strainsensitivity in response to applied strain or thermal loading, with aresolution proportional to 1/Q.

As compared to conventional fiber Bragg grating sensors, microringresonators are significantly smaller in size (e.g., between about 1-10μm for some embodiments of microring resonators versus between about125-250 μm for Bragg grating sensors) and permit spatial resolution ofat least two orders of magnitude better. In addition, in someembodiments, the quality factor Q of a microring resonator, typicallyapproximately 10³ to 10⁴, can be, e.g., a few times better, or even tentimes or more better, than conventional fiber Bragg grating sensors,thus improving strain resolution. Microring resonators, likenanophotonic crystal resonators, have temporal resolution on order25-100 ps. Current detector technology permits measurement of modulationrates up to 10¹¹ Hz, and thus the temporal resolution measurable by thesystem is limited mainly by the photon lifetime in the resonator. Thispermits significantly improved temporal sensing and detection ofhigh-frequency small-amplitude strain variation, which is supported evenwith commercial off-the-shelf photodetectors.

As compared to the 1-dimensional nanophotonic crystal resonators shownin FIG. 1( a), microring resonators are inherently 2-dimensionalstructures. Hence strain measurements from a single microring resonatorwill yield the aggregate in-plane strain at its position. To achievestrain directionality measurements, a differential structure 320 withmicrorings in approximately orthogonal directions, as illustrated inFIG. 3( b), can be employed. Although in some embodiments microringresonators sense with slightly lower spatial resolution and slightlylower strain sensitivity than nanophotonic crystal resonators, theoptical coupling losses to microring resonators can be lower than fornanophotonic crystal resonators, allowing for potentially easiercharacterization and measurement. In some embodiments, ring resonatorscan have a Q between about 2,000 and about 20,000. In some embodiments,nanophotonic crystal resonators have a Q of about 70,000. In general,photonic resonators can have a Q between about 100 and about 10⁵.

Higher order resonator structures, such as structures 300 that shown inFIG. 3( a), can be used to help to improve the sensitivity of thesensor. Larger microrings, with larger radii, can be used to obtainlarger Q resonances for higher sensitivity.

In some embodiments, photonic sensors, e.g., nanophotonic crystal and/ormicroring resonators, are integrated into a short closed-loop opticalwaveguide (e.g., having a perimeter of approximately 50 mm˜100 mm) withan input-output coupling structure.

Note that using optical fiber input and output coupling to theresonators provides data transmission at the speed of light between theindividual sensors and the data collection endpoints in the sensornetwork. This provides distributed sensors with sufficiently high speedsfor demanding applications, for example, determining information aboutcritical locations of an aerospace structure, or about the high-speeddynamics of fracture or ballistic impact. Since the photonic sensors arefabricated on-chip, computational and decision logic transistors fordeployment of protective systems or corrective actions, for example, canbe integrated on-chip for minimal time-delay.

Photonic sensors can be defined through standard semiconductorlithography techniques, permitting low-cost batch fabrication as well ashigh precision location of the sensors on-chip. This provides for thedevelopment of an on-chip parallel sensor array. As described above,signal light can be introduced to the sensors via an optical fiberattached to the chip, and a detector can collect light returning fromthe sensors via a second optical fiber attached to the chip. As shown inFIG. 4, information from multiple arrays of the photonic sensors can becollected over the same fiber bus through wavelength multiplexing.Specifically, in some embodiments a single global light source can beinput to an array of microring resonators via an integrated waveguide(curve below resonators), a part of which is adjacent the resonators inthe array. As the light traverses the waveguide, it couples to eachresonator in the array as it passes, providing an input to theresonator. Similarly, as each resonator produces an output, that outputcouples to an integrated waveguide (curve above resonators), a part ofwhich is adjacent the resonators in the array, and which feeds theoutput to a single detector. The output relates to the responses of thedifferent resonators in the array. Commercial off-the-shelfphotodetectors can have nanosecond or better response times, supportingsignificantly improved temporal sensing and detection of multiplehigh-frequency small-amplitude signals from the resonators in the array.

The photonic resonators can be designed using a variety of analytic andnumerical techniques, including finite difference method (FDM) modesolvers, finite difference time domain (FDTD) calculations, andcoupled-mode theory. With high index contrast waveguides, theseresonators can be made very small since radiation losses from tightbends are greatly reduced. These photonic crystal and microringresonators can be side-coupled or vertically coupled to the input andoutput waveguides through evanescent fields. The resultant combinationis a system with a transfer function that has a single-pole response,i.e., that has a single central resonance frequency, and narrowbandwidth. The location of the pole, and hence the resonator Q andresonant frequency, is determined by the input-output coupling ratiosand the attenuation associated with one optical transit phase within theresonator. These parameters are optimized with analytic and numericaltechniques prior to fabrication.

Photonic sensors can be fabricated on-chip with silicon-on-insulatorwafer substrates (hence permitting CMOS-level batch processing) and thechips packaged to readily couple to signal (input) and detectorsingle-mode optical fibers. For example, microring resonators can befabricated in high index contrast silicon nitride and silicon materialsystems using standard CMOS processes. In one method of making microringresonators, a photoresist, e.g., PMMA, is coated over the siliconmaterial from which the sensor is to be made. Electron-beam lithographyis then used to define a pattern in the PMMA. The PMMA is developed andsubsequently metallized, with a liftoff step, to leave a patterned metalmask over the silicon material. The assembly is etched through theentire depth of the silicon material, and slightly into the insulatorlayer of the substrate. Then the metal mask is removed using standardCMOS processes to leave the microring resonator sitting on a “pedestal”of insulator, which can help to optically isolate the sensor.

Recent advances in photonic device fabrication have made it possible toconstruct very small microring resonators in a variety of materials,including glass, polymers, silicon, silicon nitride (Si₃NH₄), siliconoxynitride (SiON), and III-V semiconductors. The choice of materialdepends upon the particular application. For example, in optical signalprocessing applications like wavelength conversion and high speedoptical switching, materials with high nonlinear coefficients such asAlGaAs/GaAs can be used.

In addition, note that in some embodiments, the photonic resonatorsensors, in the planar semiconductor fabrication process described here,can be designed for operation in only one light polarization (eithertransverse electric or transverse magnetic). Hence, the effect ofstochastic polarization clue to mechanical strain on signal input-outputoptical fibers might affect operation of the designed subwavelengthresonators and shift the resonance peak slightly. This polarizationdependence, however, can be largely removed using customized commercialpolarization maintaining (PM) fibers that adequately maintain the samepolarization over lengths of 1 kilometer or more.

In some embodiments, distributed microring and nanophotonic crystalresonator sensors are selectively embedded at sensitive locations withina piece, such as a computer chip or mechanical product, withoutinterfering with normal operation of the structure. This helps to avoiddirectly exposing the sensor to external manufacturing environments, forexample, chemicals, moisture, and contamination. Many structuresfabricated in hostile manufacturing environments are metallic. Thusembedding sensors in metal is one method of protecting them while usingthem in a hostile environment.

One example of a procedure for fabricating and embedding sensors in ametal in accordance with some embodiments is schematically depicted inFIG. 5. First, at (1), Si₃N₄ is deposited using low-pressure chemicalvapor deposition (LPCVD) on a Si substrate. As shown in FIG. 5, theSi₃N₄ is coated on both sides of the substrate. The Si₃N₄ facilitatesthe release of the sensors from the substrate later during theprocedure. Plasma enhanced chemical vapor deposition (PECVD) is thenused to deposit a first layer SiO₂, which acts as an optical insulatorfor the sensor, and then a layer of Si_(x)N_(y), e.g., the waveguidematerial, over the LPCVD Si₃N₄ film. In general, Si_(x)N_(y),SiO_(x)N_(y) or another suitable material can be used as the waveguidematerial, as discussed above and as known in the art. In this example,the sensors are photonic devices made with the PECVD Si_(x)N_(y) film,and have an optically insulating structure formed of SiO₂. In general,any sensor, such as the photonic sensors described herein can beembedded in metals using the described procedure.

Next, at (2), optical/electron-beam lithography and reactive ion etching(ME) are used to define the photonic sensors within the waveguidematerial. In some embodiments, this is done by using conventionallithographic patterning of photoresist (PR) as a mask material, and RIEis subsequently performed to etch the waveguide material in accordancewith the pattern of the PR mask.

Next, at (3), the PR is stripped after the etching is complete and SiO₂is then deposited using PECVD. This oxide layer, as well as the onedeposited at (1), are used to optically insulate the sensors from themetal layers.

Next, at (4), The SiO₂ is patterned using optical/electron-beamlithography and RIE as described above, and as is known in the art. Tiand Ni are optionally sputtered over the resulting structure as adhesionlayers.

Next, at (5), Ni is electroplated over the structure. The Ni layer isrelatively thick, e.g., about 1 μm, and is structurally robust. TheLPCVD Si₃N₄ at the bottom of the substrate is then dry etched from theback side using RIE. Wet etching of the silicon substrate is then done,followed by an RIE etch of the remaining Si₃N₄ layer immediately beneaththe sensors. This frees the sensors from the Si substrate, effectively“transferring” them to the Ni, which then acts as a new “substrate.” Tiand Ni are then optionally sputtered as adhesion layers over the newlyexposed SiO₂ surface, which previously was attached to the substrate viathe Si₃N₄ layer. Ni is then electroplated over the patterned structure,substantially embedding the sensors, plus the dielectric layers, withinthe metal. The sensors, thus embedded, can then be readily coupled to apiece for which measurements are desired, for example a computer chip ormechanical product. For example, in some embodiments, a material forwhich measurements are desired is grown on top of the sensor assembly.

Methods and materials other than those described can be used. Forexample, electroplating can be selected to deposit metals for thisapplication because it works at near room temperature and generatesalmost no stresses. However, other metal deposition processes canadditionally or alternatively be used, for example laser deposition.Other metals, such as stainless steel, can also be used. In manyembodiments, the metal is compatible with standard CMOS process steps.

The number of sensors and their spatial distribution determine the widthand length of the embedding thin metal layers. Functionally gradientthin film structures can be designed, selected, and fabricated tooptimize material properties such as thermal conductivity, thermalstability, diffusion compatibility, strength, and thermal expansion,between optical layers (e.g., SiO₂) and metal layers (e.g.,electroplated nickel). Appropriately selecting properties betweenadjacent layers can, among other things, discourage delamination of thelayers and improve the performance of the device. With proper choice ofcomponents and individual layer thicknesses, the property of amultilayer coating can be tailored over the average of the twocomponents. The properties of different materials are well studied anddocumented. However, the growth conditions of the materials (such astemperature, pressure, and deposition rate) can also influence theproperties of the as-grown materials. As is known in the art, modestexperimentation can be performed in order to obtain “recipes” forgrowing materials that are suitable for the particular application.

Optical sensors, such as SiO₂-based Fiber Bragg Grating (FBG) sensors,can be successfully embedded into metals, e.g. nickel and/or stainlesssteel for temperature and strain measurements in manufacturingprocesses, as shown in FIGS. 6( a) and 6(b). Single thin filmthermocouples (e.g., type K: alumel-chromel) can also be embedded intometal, as shown in FIG. 6( c). The embedded sensor can be annealed at800° C. for 3 hrs in argon. X-ray Photo-electron Spectroscopy (XPS)before and after annealing can be used to show that the integrity of thedielectric thin film Al₂O₃ and Si₃N₄ layers are well preserved,indicating that photonic sensors fabricated from these materials andembedded in metal will also survive. The sensor pads on thethermocouples can be soldered with fine electrical wires, which can thenbe connected to a PC-based data acquisition system for calibration in atemperature-controlled oven.

Sub-micron optical thin films demonstrate improved properties over theirmacro-optic counterparts. For example, amorphous submicron/nano SiO₂ andSi₃N₄ thin films have significantly higher strain limit (3˜10%) thanthat of most metals (0.2˜4%). In some embodiments, the core regions ofthe optical waveguides can be further optically isolated from the metalsurfaces in which they are embedded in order to avoid unacceptably largeoptical losses due to optical absorption by the metal. This isolationcan be achieved by using a cladding layer (for example, SiO₂) ofadequate thickness so that the evanescent waveguide field will besufficiently weak at the cladding-metal interface. Standard analytictools from integrated optics can be used to compute the appropriatecladding thickness. Single-mode waveguides cladded on both sides with a2.0 μm silicon oxide show negligible loss into the substrate.

FIG. 7 shows images of the transfer of a batch of photonic sensorsdirectly from a silicon wafer onto an electroplated nickel layer, andthe embedding of the sensors in metal. The process steps described abovein connection with FIG. 5 can be applied to a wafer having thin filmspatterned across its surface using the batch fabrication ofmetal-embedded photonic sensors in a clean room environment usingstandard CMOS processes. Other kinds of structures, e.g., optical andelectronic devices, can also be embedded in metals using the described,or similar, techniques.

As mentioned earlier, the photonic sensors described herein respond tostrain caused by both mechanical and thermal effects. FIG. 8 is aschematic of a system for enhancing the sensitivity of microringphotonic sensors to mechanically induced strain by providing a thermalreference scheme, in accordance with some embodiments. The systemincludes: a first microring resonator 810; a second microring resonator820, which is suspended on a cantilever 830; a signal waveguide 840; andan output waveguide 860. Signal waveguide 840 is split by a “y-splitter”into first and second signal waveguides 841 and 842, which lead to firstand second microring resonators 810 and 820, respectively, substantiallyas discussed above relative to the single microring resonators. Outputwaveguide 860 is split by a “y-splitter” into first and second outputwaveguides 851 and 852, which lead from first and second microringresonators 810 and 820, respectively, substantially as discussed aboverelative to the single microring resonators. The system also includes anoptical source (not shown) that is coupled to signal waveguide 840 (andthus to first and second signal waveguides 841 and 842), and a detector(not shown) that is coupled to output waveguide 860 (and thus to firstand second output waveguides 851 and 852).

First microring resonator 810 experiences both mechanical and thermalstrain effects arising from its environment. However, second microringresonator 820 is suspended on cantilever 830, and thus does notexperience significant mechanical strain due to, e.g., compressiveforces F_(compression) on the system, although it remains thermallycoupled to the system and thus experiences thermal strain effects. Insome embodiments, signal light from an approximately single-wavelengthoptical source is split at the “y-splitter.” A portion of the signallight is input to the first microring resonator 810 via first signalwaveguide 841, and another portion of the light is input to the secondmicroring resonator 820 via second signal waveguide 842. Because thefirst and second microring resonators 810 and 820 experience differentstrain effects, they couple differently to the signal light, e.g., withdifferent coupling efficiencies. The light that couples to each of themicroring resonators then couples as output into the corresponding firstor second output waveguide 851 or 852. Where the first and second outputwaveguides join at the “y-splitter,” the respective outputs from thefirst and second microring resonators 810 and 820 interfere with eachother, when the optical path lengths (841+810+852) and (842+820+851) aredesigned and fabricated to be sufficiently similar to permitinterference between the output signals. The thermal signal 862 from thesecond resonator 820 destructively interferes with the thermal componentof the overall (thermal plus mechanical) signal 861 from the firstresonator 810, such that the main remaining signal component is themechanical signal as measured by the first resonator. Some embodimentsinclude a subsystem for adjusting the relative optical phases of theoutput signals from the two resonators in order to enhance theirdestructive interference, optionally under control of the computerprocessor. Note that in the illustrated embodiment, the direction ofstrain is orthogonal to the direction of light propagating through thesystem, although other configurations can be used.

In other embodiments, the first and second microring resonators 810 and820 are designed and fabricated so as to have slightly differentresonant wavelengths, and the signal light includes first and secondwavelengths that are at or near the resonant wavelengths of therespective resonators. After the signal light couples to the resonators,the resulting output signals are joined together by the “y-splitter” andsent to the detector, where the thermal signal as measured by the secondmicroring resonator 820 are normalized away, or otherwise subtractedfrom, the overall signal (thermal plus mechanical) as measured by thefirst microring resonator 810.

In general, different embodiments of photonic sensors, e.g., thenanophotonic crystal and microring resonators described herein, or othertypes of sensors, can be implemented in this scheme, and otherinterferometric or other kinds of input/output arrangements arepossible.

In summary, photonic sensors, such as nanophotonic crystal and microringresonators, can be individually designed for a particular application,and can have strain sensitivities on the order of 10⁻⁵. The strainsensitivity is inversely proportional to Q, and hence the sensors aredesigned to have as large a Q as possible. In some embodiments, photonicresonators are designed to operate in an interference scheme with twocoupled resonators, and, thus, have an even higher strain sensitivity,e.g., up to order 10⁻⁸ strain. The photonic sensors can be fabricatedusing standard CMOS processing, and can be embedded in metals tooptimize their performance in hostile manufacturing environments. Thephotonic sensors can be used to measure in-situ strain and temperatureeffects on computer chips and mechanical elements as they are beingfabricated, and can also be used to measure residual strains in thechips and elements after fabrication is completed. Although nanophotoniccrystals and microring resonators of a particular design are describedherein, other structures that serve as photonic sensors, and that canoptionally be embedded in metals, can also be used in some embodiments.

In view of the wide variety of embodiments to which the principles ofthe present invention can be applied, it should be understood that theillustrated embodiments are illustrative only, and should not be takenas limiting the scope of the present invention. Features of the presentinvention can be used in any suitable combinations.

1.-11. (canceled)
 12. A method of embedding a photonic sensor in metal,the method comprising: depositing at least one first opticallyinsulating layer on a substrate; depositing and patterning at least onewaveguide material on the first optically insulating layer to define thephotonic sensor; depositing and patterning at least one second opticallyinsulating layer over the photonic sensor; depositing at least one firstmetal layer over the at least one second optically insulating layer;etching the substrate to free the first and second optically insulatinglayers, the photonic sensor, and the at least one first metal layer fromthe substrate so as to expose a surface of the at least one firstoptically insulating layer; depositing at least one second metal layerover the exposed surface of the at least one first optically insulatinglayer so as to substantially embed the photonic sensor and first andsecond optically insulating layers between the first and second metallayers.
 13. The method of claim 12, wherein patterning the waveguidematerial comprises at least one of lithography and etching. 14.-22.(canceled)
 23. A system for sensing at properties of a workpiece, thesystem comprising: a first arrangement which is configured to output atleast one optical signal; a second arrangement, coupled to the workpieceand to the optical signal, configured to generate at least one outputsignal in response to the optical signal, wherein at least one of anintensity of the output signal and a wavelength of the output signaldepends on at least one of thermal characteristics and mechanicalcharacteristics of the workpiece; a third arrangement which configuredto detect the at least one output signal; and a third arrangement whichconfigured to determine at least one of the thermal characteristics orthe mechanical characteristics of the workpiece based on the detected atleast one output signal.
 24. The system of claim 23, wherein the secondarrangement comprises at least one of a microring resonator or a defectin a photonic crystal.